Certain scientific instruments, such as airborne particle counters, use a vacuum pump to sample aerosols by pulling an air stream through the sampler. The vacuum pump can be built-in or connected externally. The vacuum pump draws the sample air into the particle sensor where particles are sized and counted, sometimes using optics and sometimes using irradiation or other techniques. Particle counter vacuum pumps have unique requirements. They must be reliable over a long period of time, support flow control typically using DC power, have little or no flow pulsations for continuous flow, and they must not shed particles.
Reciprocating pumps, such as diaphragm or piston pumps offer long life and flow control. These pumps can also produce a high vacuum by positive displacement of air. However, reciprocating pumps have excessive flow pulsations and they shed particles. Consequently, reciprocating pumps have had limited success in the particle counter industry.
Rotary vane pumps offer high vacuum and good flow control. However, these pumps have limited life because vanes fail. They also have flow pulsations and they generate particles.
Centrifugal pumps or blowers offer long life, flow control and continuous flow. However, these pumps have a weak air flow characteristic due to low vacuum. They also generate particles.
Most pump technologies produce flow pulsations or surges in the sample flow path. These pulsations degrade particle sensor resolution and therefore reduce particle counter measurement accuracy. Also, most pump technologies generate particles inside and outside the flow path. Particles inside the flow path can easily be filtered with an exhaust filter. Particles shed outside the flow path, such as in motor brushes, bearings, and moving parts of the pump are problematic. These particles contaminate the sample aerosol when they escape through vents or gaps in the instrument enclosure. An object of the invention is to devise a pulsation and particle suppressing air flow system for an air sampling instrument.
Efforts have been made in the prior art to establish a continuous smooth volumetric flow through an air sampling instrument. For example, in U.S. Pat. No. 5,295,790 C. Bossart et al. teach a portable sampling pump apparatus including a flow control mechanism having a flowmeter for feedback control for the pump motor. The pump apparatus includes an electric motor, a pump operably driven by the electric motor, a laminar flow element positioned in a flow path of the pump, a pressure transducer for sensing a pressure drop across the laminar flow element and for producing an electrical signal that is directly and linearly proportional to the volumetric flow rate through the pump, and a motor control circuit which uses the electrical signal to control the voltage applied to the motor and to thereby regulate the flow of the pump.
In U.S. Pat. No. 5,892,160 P. Hall teaches an air or gas sampling device that utilizes a small tube calibrated under isothermal conditions. The relationship of pressure at the input end of the tube to flow is plotted and stored in a microprocessor for comparison with pressure monitored during sample pumping. Pulsations in flow caused by the air or gas pump can be neutralized by a damper comprising a small chamber having a diaphragm for one wall.
An object of the invention was to devise an improved continuous air flow system for an air sampling instrument with suppressed pulsations and particle release.